1. Field of the Invention
The present invention relates to improvements in threadless couplings for pipes for connecting steel or plastic pipes such as electrical conduits, water-service pipes, steam pipes, and plant pipes.
2. Description of the Prior Art
In so-called electrical conduit laying operations for laying conduits in which insulated electrical wires or cables are placed, couplings are used to connect conduits of a fixed length, the number of which is determined by the length of piping to be laid. A coupling in conventional use comprises a tubular coupling provided with internal threads which is connected by screwing on to an externally threaded end of a conduit. This type of screw-on coupling is unusable when a pair of curved or bent conduits need to be connected. This is due to the fact that the connecting operation must be typically held in a narrow place and at least one of the conduits to be connected must have sufficient room to allow it to be rotated and screwed onto the other conduit.
To obviate this inconvenience, threadless couplings capable of connection without rotating the conduit have been in use. A prior-art threadless coupling for thin-steel conduits which is specified in the Japanese Industrial Standards (JIS) is shown in FIGS. 1A and 1B. In these drawings, coupling is performed by inserting the ends of the conduits into both ends of a cylindrical coupling body 1 until the end of each conduit comes in contact with a pipe stopper (not shown) and by tightening the tightening screws 2. This coupling gives rise to the following problems: the tightening of the tightening screws 2 is time consuming, resulting in a poor operation efficiency; threads of the tightening screws 2 are often likely to be stripped or crushed; and the conduits, being fastened at two points by the tightening screws 2, cannot be maintained straight.
A threadless coupling illustrated in FIG. 2 has also been used for connection of gas pipes, water-service pipes, and conduits. This threadless coupling has a cylindrical coupling body 5, a pipe stopper 3 at the center of its inner periphery and external threads 4 on the outer periphery of both of its ends, a pair of rubber rings 6, a pair of metal cut rings 7, and a pair of cap nuts 8 for pressing the rubber rings 6 and the cut rings 7 against the end of the coupling body 5.
When this threadless coupling is used to connect electrical conduits, first the cap nuts 8, the cut rings 7 and the rubber rings 6 are inserted (in the order recited) over the ends of both of the conduits. Then the ends of the conduits are inserted into the coupling body 5 such that these ends will be butted. Finally, the cap nuts 8 are tightened such that the cut rings 7 and the rubber rings 6 are pressed against both ends of the coupling body 5 and deflected inwardly into contact with the conduits.
However, this type of coupling has a complicated construction and, therefore, requires a high manufacturing cost. Also, the connection of this coupling requires the assembling and tightening of many parts by specific procedures, which is also time consuming, thus resulting in a low operational efficiency.
Furthermore, a prior-art threadless coupling shown in FIGS. 3A, 3B and 3C has been proposed in a Japanese utility model application. This threadless coupling has a cylindrical body 9 formed in a cylindrical shape from a flat square metal-plate having opposite edge sections which can be brought close to each other. In these opposite edge sections of the cylindrical body 9 are formed hook-like engaging sections 10 and 11 which are engageable with each other. As shown in FIG. 3C, in the inner peripheral surface of this cylindrical body 9, two projections 12 are formed to bite the conduits to effect fastening.
When the conduits 13 are connected, the conduits 13 are inserted as deep into the central section of the cylindrical body 9 from the opposite ends thereof. Then the cylindrical body 9 is closed until the engaging sections 10 and 11 are engaged with each other as shown in FIGS. 3B and 3C.
However, since a considerable force is required to engage the engaging sections 10 and 11, there exist the following problems: a special tool or device is needed so that the connecting operation is not simple; the coupling is unusable for thin steel conduits because of the use of the engaging sections 10 and 11; and the conduits, once connected, cannot easily be disconnected.